1. Field of the Invention
The present invention relates to cationic coating compositions, more particularly, to cationic coating compositions containing an onium group-containing polymer in combination with an aminoplast curing agent, and to the use of these compositions in cationic electrodeposition.
2. Brief Description of the Prior Art
Electrodeposition is a coating application method which involves the deposition of a film-forming composition under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection and low environmental contamination. Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today it is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition. Other areas of application are primer coating or one-coat topcoating of automobile accessories, farm machinery, house and electrical appliances, steel furniture and various structural components. The first cationic electrodepositable compositions used amine salt group-containing resins or onium salt group-containing resins as the binder, see, for example, U.S. Pat. No. 3,454,482 to Spoor et al and U.S. Pat. No. 3,839,252 to Bosso and Wismer. The curing agents for these resins were usually aminoplasts since these curing agents were used quite successfully with the earlier anionic electrodepositable resins. However, it was initially found that the aminoplasts were not completely satisfactory for use in cationic electrodeposition. Aminoplasts cure best in an acidic environment. With anionic electrodeposition, this poses no problem since the anionically electrodeposited coating is acidic. However, the cationically electrodeposited coating is basic and relatively high temperatures, that is, about 400.degree. F. (204.degree. C.) or higher must be used for complete curing of the cationically electrodeposited coating. There have been attempts to overcome this problem such as by trying to incorporate acid catalyst into the deposited film, but these measures have, in general, not been satisfactory.
It has now been found that a particularly structured aminoplast provides for low temperature curing. Surprisingly, it has been found that the low temperature curing ability of the aminoplast seems to be particular to onium group-containing resins, particularly quaternary ammonium group-containing resins. The aminoplast does not, for example, cure, at low temperature, amine salt group-containing resins which are deposited by cationic electrodeposition.